1. Field of the Invention
The present invention relates to an infrared optical limiter, and more specifically, to a passive broadband infrared optical limiter device based on an array of micro-optomechanical cantilevers that bend as a result of thermo-mechanical forces.
2. Description of the Related Art
There is a need for a broadband passive infrared optical power limiting device. In the present application, the wavelengths of the infrared spectrum will be referred to as the mid wavelength IR (mid IR, wavelength 3 to 5 microns) region and the long wavelength infrared (LWIR, wavelength 7 to 14 microns) region. Such an infrared optical limiter would be useful for protecting light sensitive items, such as the human eye, a photodetector or a camera against unexpectedly strong illumination. An ideal optical limiter is transparent under low level illumination at the wavelength of interest, but “dark” under strong incident light power. The optical signal power transmitted through an optical limiter is ideally constant, irrespective of the incident power, when the incident signal power is higher than optical limiter threshold. It is also desirable for an optical limiter to have a low initiating threshold and broad spectral range.
Optical limiting techniques using reverse saturable absorbers (RSA) solutions and multi-photon absorbers dyes are well known in the art. The disadvantage of these materials as limiters includes a high incident fluence threshold which the optical limiting behavior begins. More importantly, however, there are no known RSA or multi-photon solutions that can be used in the mid IR and LWIR regions.
Others have proposed using a vanadium oxide interference mirror for optical limiting in the infrared region. See for example, O. P. Konovalova, Al. Sidorov, “Interference systems of controllable mirrors based on vanadium dioxide for the spectral range of 0.6–10.6 μm”, J. Opt. Technol., 66(5), p. 391 (1999). The Konovalova et al. device is based on a vanadium oxide film that absorbs incident laser energy and changes phase. The main function of this limiter is laser hardening, and it is a relatively narrowband device that is not suitable for handling a broadband of wavelengths. It has an initiating threshold on the order of 1 MW/cm2, which is too high for the mid IR and LWIR range. A suitable mid IR and LWIR range limiter must limit a continuous wave source, having a broad band spectrum (continuously from 3 to 14 microns wavelength), but having relatively low peak power (˜1 W/cm2) radiation.
Others have proposed using microcantilever devices to make an infrared focal plane array sensor. See for example, P. I. Olden, and et. al., “Uncooled thermal imaging using a piezoresistive microcantilever,” Appl, Phys. Lett. 69 (21), 3277 (1996); and T. Perazzo, and et. al., “Infrared vision using uncooled micro-optomechanical camera,” Appl., Phys. Lett., 74(23), 3567 (1999). Another example of microcantilever devices is disclosed in U.S. Pat. No. 6,392,233 (Channin et al.) which is entitled “Optomechanical Radiant Energy Device”. The Channin et al. device is a radiant energy detector built from microcantilever devices. Although the use of microcantilever devices as a detector is well known in the art, the use of microcantilever devices as an infrared limiter appears to be unknown in the art.